Fred,
Well its not precession !
Thats about 33 arc mins per century not every 4 months.
So its most likely ground movement
Is the Obs situated on or adjacent to any backfill or terraformed land ?
Have you noticed if its in the same direction or if it varies back and forth with Winter and Summer ?
As the ground gets wet the soil (especially clay) expands, if your pier is between two areas that are exposed to moisture differently then that could be a possible reason.
Before you go digging up the Obs - it may be possible to change the dyanamics of your soil by adding some extra soil drainage - put in some agricultural draining pipe (convoluted poly pipe with lots of holes in it) and try to remove all the excess water.
I have done this and its amazing just how much water comes out.
Fix up any backyard drainage issues - eg off the obs roof, shed/house gutters, neighbours run off etc.
Also is there any disturbed soil nearby - pool, sewer, drainage, water mains, underground electricty, old tree stump from original site clearing etc etc - these could all be causing soil settling/movement many years after the work was done.
But bigger deeper footings are better - however if your soil is moving it may be the case that going deeper will not of itself necessarily solve the problem.
On the other hand repeatedly leaning against the OTA in the same place night after night after a few beers may be the culprit !
I would look at drainage and water runoff first - followed by light beer
Cheers
Rally